High capacity hard disk drives (HDDs) and other magnetic storage devices employ thermally stable fine-grained high coercivity media. The high coercivity media requires write fields in excess of those attainable with current write heads, the performance of which are limited by pole tip saturation and material properties. Energy-assisted magnetic recording techniques overcome the coercivity/write-field conflict. In heat-assisted recording (HAMR), the temperature of the medium in the write zone is elevated to near the Curie point, typically by a laser, easing magnetization by an achievable write field. An alternative method is microwave-assisted magnetic recording (MAMR), in which near-field microwave radiation excites the recording medium at its ferromagnetic resonance frequency, permitting magnetic moment polarity-switching with reduced write fields. The frequency of the near-field microwave radiation is in the 10 GHz-60 GHz range and is provided by a nanometer-sized spin-torque oscillator (STO) fabricated in the write head gap.
However, STO behavior is adversely affected by the cyclically-reversing write-gap field, and as such, it is desired to control the STO current so as to improve the MAMR write process. It is also desired to measure and/or to align the STO frequency to the ferromagnetic frequency of the recording medium. Knowledge of STO frequency is desired in 3-D recording schemes in which a multilayer recording medium employs layer-specific ferromagnetic frequencies to select layer access. Still further, it is desired to determine whether the STO is oscillating in a stable manner because instability can be caused by fields emanating from prior-recorded information.